From a perspective of preserving the environment, wind turbine generators using wind power (a wind turbine) are becoming popular. In a wind turbine generator, kinetic energy of the wind is converted into rotational energy of a rotor and the rotational energy of the rotor is then converted into electric power by a generator. Normally, the wind turbine generator is connected to the grid and electric power generated in the wind turbine generator is supplied to the grid.
It is know that a frequency of the grid is determined based on a demand-supply balance of electric power. More specifically, when power generation exceeds power demand, the surplus energy is stored as kinetic energy in generators that are connected to the grid, hence increasing their rotational speed, i.e. the grid frequency. In contrast, when power demand exceeds power generation, kinetic energy is extracted from the generators to compensate for power shortage in the grid, hence decreasing the rotation speed of the generators, i.e. the grid frequency.
Therefore, some of the electric power plants operate to follow a change in the power demand (a load-follow operation) to maintain the overall demand-supply balance in the grid. The electric power plants that perform the load-follow operation are normally thermal power plants as it is relatively easy to control their output by adjusting an amount of fuel supply. In this manner, some of the thermal power plants function as an adjusting power for maintaining the demand-supply balance. In contrast, power plants such as nuclear power plants and hydraulic power plants are difficult to control output (output power) in a short period of time and thus, those power plants function as a base supply power for continuously supplying a constant amount of electric power.
Herein, if the proportion of the wind turbine generators that have significant power variations is high among the entire generators connected to the grid, the adjusting power by the thermal power plants may not be sufficient. Particularly, in the nighttime when the power demand is extremely low compared to the daytime, the output of the thermal power plants are already set near the minimum output and thus, it is difficult to handle an increase in the output of the wind turbine generators.
Therefore, in the case where the adjusting power of the grid (a reduction margin) is insufficient, an upper limit of the output of the wind turbine generators are, for instance, set by a command from an external dispatch center.
Although not directly related to setting of the output upper limit of the wind turbines, there are known methods for controlling a ramp rate of a wind farm together with a rate of change in output of power plants including the thermal power plants as the adjusting power (see Patent Literatures 1 to 3). This method controls the ramp rate of the wind farm to the rate of change on the grid side as it is difficult to rapidly change the output of the power plants including the thermal power plants being the adjusting power in response to sudden change in the output of the wind farm due to wind conditions.